Composition structure for NOx sensors

ABSTRACT

A structural arrangement for NO x  sensors including a first selectively oxygen ion-conductive layer and a second gas-permeable, nonconductive layer, the latter having a porous spinel structure or porous platinum. The oxygen ion-conductive layer made from a mixed-conductive ceramic can have an additional layer of a material which is catalytically inactive to NO x .

FIELD OF THE INVENTION

The invention relates to a structural arrangement for NO_(x) sensorsincluding a first selectively oxygen ion-conductive layer and a secondgas-permeable nonconductive layer.

BACKGROUND INFORMATION

It is known that exhaust gases of internal combustion engines, leanmixture engines in particular, contain nitrogen oxides and other gasesin addition to unburned fuel components and oxygen. The composition ofthe exhaust gas is essentially determined by the setting of the air-fuelmixture with which the internal combustion engine is operated. If, forexample, fuel is present in excess of a stoichiometric mixture,considerable amounts of unburned or only partially burned fuel arepresent in the exhaust gas while oxygen in excess of a stoichiometricmixture of the air in the air-fuel mixture results in a correspondinglyhigher concentration of oxygen in the exhaust gas. A known method forsetting an optimum air-fuel mixture is to determine the composition ofthe exhaust gas in a proportional probe. In doing so, the limitingcurrent probe has a solid electrolyte arranged between two electrodes,one electrode being exposed to the exhaust gas via a diffusion barrier.With a constant voltage applied to the electrodes, a limiting currentoccurs on both electrodes due to a difference in oxygen concentration,the limiting current being measured by a measuring device and analyzed,for example, for setting the air-fuel mixture with which the internalcombustion engine is operated.

One possibility for determining oxygen and nitrogen oxides in particularin gas mixtures is described in the article by N. Kato, K. Nagakaki andN. Ina, SAE 1996, pages 137 to 142. The reactivity of the nitrogenoxides and other gas components with parts of the electrode is regardedas one of the disadvantages of the method described in that article.

Moreover, unpublished German Patent Application No. 196 35 494.3(equivalent to German Published Patent Application No. 196 52 968)describes a measuring arrangement for the determination of gascomponents in gas mixtures with at least one electrochemical solidelectrolyte measurement cell and at least one cathode exposed to the gasmixture to be measured, the cathode being spatially separated from alayer that selectively conducts oxygen ions by an electricallynonconductive layer which is gas-permeable at least in areas.

Consistent with this patent application, a mixed-conductive ceramicmembrane with catalytic inactivity to nitrogen oxides and electricallyinsulated via a solid electrolyte pump cell must be provided for thestructure of an amperometric NO_(x) sensor. The ceramics used for thispurpose may be composed of a metallic oxide, with perovskite ceramicsbeing used frequently.

Previously, the mixed-conductive ceramics were imprinted onto a porousAl₂O₃ structure as a support material. In this process, severalcomponents of the metallic oxide ceramic diffused into the Al₂O₃structure and formed phases with a spinel structure at the boundarysurface of the membrane, with these phases being nonconductive to poorlyconductive for ions.

An additional problem was that the ceramic materials known to beoutstandingly mixed-conductive have as a rule, however, a highlycatalytic activity toward NO_(x). On the other hand, the materials thatare inactive to NO_(x), nickel catalyst materials, show only a slightresultant oxygen permeation rate.

SUMMARY OF THE INVENTION

An object of the present invention is to make a structural arrangementfor NO_(x) sensors available in which the supporting material does notsubstantially inhibit the exchange of oxygen into the gas phase and doesnot influence the structure of the mixed-conductive ceramic andaccordingly the ionic conductivity. Moreover, the mixed-conductivemembrane should have a high electron and ion conductivity, resulting inhigh oxygen permeability while at the same time, however, the catalyticactivity to NO_(x) should be as low as possible.

The object is achieved according to the present invention by astructural arrangement for NO_(x) sensors which includes a firstselectively oxygen ion-conductive layer and a second gas-permeable,nonconductive layer which has a porous spinel structure or porousplatinum.

Spinels are understood to be a group of minerals and syntheticallyproduced materials having the general formula AB₂X₄, A signifying abivalent metal, B a trivalent or quadrivalent metal and X=O, S, Se.Al₂O₃. MgO and/or Co₃O₄ may be used as particularly preferred spinels.The use of spinels according to the present invention possibly increasesthe standard reaction enthalpy for the formation of products frommixed-conductive perovskites and from the spinel substrate structure inrelation to the previously used Al₂O₃ substrates. It might thus bepossible to inhibit the formation of nonconductive boundary phases.

Platinum substrates may be used in the production of themixed-conductive layer. In doing so, no reaction takes place between thesubstrate and the ceramic. However, the porous platinum ensures a goodbond between the substrate and the mixed-conductive layer.

In one embodiment of the present invention, the structural arrangementis characterized in that the selectively oxygen ion-conductive layer ismade from a mixed-conductive ceramic having a layer on the side facingthe exhaust gas which is made from a material that is catalyticallyinactive to NO_(x).

Examples of suitable materials that are catalytically inactive to NO_(x)are Gd_(0,7)Ca_(0,3)CoO_(3-δ), Gd_(0,7)Ca_(0,3)FeO_(3-δ), compositematerials of mixtures of Ce_(0,8)Gd_(0,2)O_(2-δ) andGd_(0,7)Ca_(0,3)CoO_(3-δ).

This layer can be applied to the layer composed of a mixed-conductiveceramic with the aid of thick-film technology or by a vapor depositionmethod.

In doing so, the layer is of a nickel catalyst material which is atleast more than one layer of atoms thick, the nickel catalyst materialrendering the mixed-conductive ceramic membrane inactive on itschannel-side surface. The high ionic conductivity of the oxygen throughthe ceramic material is preserved in this manner.

In a preferred embodiment the mixed-conductive ceramic is composed of aperovskite or elpasolite, in particular having the compositionCa_(0,7)Sr_(0,7)CoO_(3-δ) or SrFeCo_(0,5)O_(3-δ).

According to the present invention, the range of variation of themixed-conductive membrane materials suitable for use is nowsignificantly and advantageously expanded by the modification of thesubstrate structure. Thus the elimination of the previous inactivationof the boundary surface distinctly reduces the permeability of themembrane with constant effective permeation quantity per surface area.Also, the entire membrane surface can be substantially reduced by thenovel structural arrangement since the permeation through the ceramicmaterial is substantially increased with continued catalytic inactivityof the surface.

Thus according to the present invention, the sensor diffusion distanceis reduced and the response time of the sensor is reduced accordingly.

It is therefore now possible in an advantageous manner to extend themeasuring range of the sensor with regard to the O₂/NO_(x) proportionwith the same membrane surface.

Moreover, the contamination of the surface advantageously results in areduction and inhibition of aging and drift phenomena, respectively, inthe ceramic membrane.

With constant oversizing of the membrane surface, which is at leastrequired for the complete oxygen removal, the total increased permeationper surface area now makes it possible for a larger proportion ofsurface area to be contaminated without disadvantageously influencingthe characteristics of the sensor.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a measuring arrangement and structural arrangementaccording to the present invention.

DETAILED DESCRIPTION

The FIGURE shows a measuring arrangement which is generally identifiedas 10. This measuring arrangement 10 has a solid electrolyte 6essentially in the form of a sheet on the side of which is arranged anelectrode which is connected as an anode 7. An electrode connected as acathode 5 is arranged on the side of solid electrolyte 6 opposite anode7, the electrolyte being made from, for example, a zirconiumdioxide-yttrium oxide ceramic. Cathode 5 is partially covered with agastight layer 3, for example of α- or γ-aluminum oxide or a layer ofpurely ion conductive material. It borders gas-permeable, nonconductivelayer 2 which has a porous spinel structure. These electricallyinsulating layers 2, 3 are covered with an additional layer 1 which iscomposed of a mixed-conductive metal oxide ceramic. This forms aselectively oxygen ion-conductive layer. The thickness of this layervaries between 5 and 200 μm. The boundary surface of mixed-conductiveceramic membrane 1 with nonconductive layer 2 is identified as 4. Layer11 which is catalytically inactive to NO_(x) is present onmixed-conductive ceramic membrane 1. Both anode 7 as well as the cathodeare connected to measuring arrangement 10 via conductors with terminals,both of which are not shown here. Electrodes 5, 7 are preferably made ofplatinum or another corrosion-resistant metal or metal alloy. Adiffusion channel 8 is arranged beneath anode 7, via which reference aircan be directed to anode 7. Diffusion channel 8 for the reference air isaccommodated in a multilayer body 9 which is built up from multiplelayers, multilayer body 9 being made from zirconium dioxide, forexample.

What is claimed is:
 1. A structural arrangement for an NO_(x) sensor,comprising: a first selectively oxygen ion-conductive layer made of amixed-conductive ceramic including a layer on a side facing an exhaustgas and formed from a material that is catalytically inactive to NO_(x);and a second gas-permeable, nonconductive layer including a porousspinel structure.
 2. The structural arrangement according to claim 1,wherein: the mixed-conductive ceramic includes one of a perovskite andelpasolite.
 3. The structural arrangement according to claim 2, wherein:the mixed-conductive ceramic includes one ofCa_(0,7)Sr_(0,7)CoO_(3-δ)and SrFeCo_(0,5)O_(3-δ).
 4. The structuralarrangement according to claim 1, wherein: the porous spinel structureincludes at least one of Al₂O₃.MgO and Co₃O₄.
 5. The structuralarrangement according to claim 1, wherein: the material that iscatalytically inactive to NOx is applied to the first selectively oxygenion-conductive layer by an implementation of one of a thick-filmtechnology and a vapor deposition operation.
 6. A measuring arrangementfor a NO_(x) sensor, comprising: a cathode; and a structural arrangementarranged over the cathode, the structural arrangement including: a firstselectively oxygen ion-conductive layer made a mixed-conductive ceramicincluding a layer on a side facing an exhaust gas and formed from amaterial that is catalytically inactive to NOx, and a secondgas-permeable, nonconductive layer including a porous spinel structure.